The treatment of deep-seated tumors by d(14)+Be neutron beam therapy is limited due to the steep decrease in depth-dose distributin. The interaction of fast neutrons in tissue leads to a thermal neutron distribution which can be used to produce neutron capture reactions with boron. In this thesis we review the Monte Carlo calculation of the effect which the neutron capture reaction occurs in fast neutron therapy. At last, we can find that the absorbed dose in a tumor with 100 ppm boron, at the same depth, is enhanced by 15%.